The overall objective of this proposal is to characterize the structure and function of VSV DI RNA. Four specific areas will be focused on. The first two, 1) "snap-back" RNA, and 2) inverted terminal repeat sequences, are related to recently discovered structural features of VSV DI RNAs. We plan to establish, using biochemical and electron microscopic analysis, whether these features are also characteristic of non-defective virion RNA of the infectious virus, and whether or not they are involved in the generation and/or interference properties of DI. The last two topics focus on the regulation of VSV RNA synthesis. We plan to characterize biochemically 3) VSV variants with altered DI polymerase activity, and 4) the role of the virion transcriptase inhibitor, both of which have recently been characterized by the PI.